The Whole Of Water Sources Are Not Rendered Unclean By Dead Creeping Things, Only The Part Where They Are (Leviticus 11:36).
“Nevertheless a spring or a pit in which is a gathering of water shall be clean, but what touches their carcase shall be unclean.”
Uncleanness does not apply to all the water in a spring or cystern, only to what is actually known to have touched the carcase. This probably meant that they must ladle out the dead rodent or reptile with the surrounding water. It would hardly have been practical in the course of life if every cystern in which a dead rodent or reptile fell had to be treated as permanently unclean in totality until emptied and refilled. Water was too scarce. And the thought was there that the quantity of water would dilute any uncleanness. Wherever people stopped for a length of time cysterns of kinds would be built for storing and catching water (compare Genesis 37:24) and they were vital for making the best use of water. They would later be an essential for living in the hill country in Canaan.